Methanol or methyl alcohol has industrially been produced for many decades. Its characteristic of burning without emitting pollutant substances such as NO.sub.x, SO.sub.x and dust when used in steam generators or gas turbines, and its property of considerably reducing CO emission when used in mixture with gasoline, make methanol an ecological energy carrier.
Methanol used as an energy carrier also has a strategic importance in that it enable coal to be economically utilized as a cleaner fuel.
The crude product of the present process is a mixture of methanol and methyl formate which has the better burning performances and is cheaper compared to a single methanol. These characteristics make the crude product of the present process a very good energy carrier.
Moreover, methanol is an important chemical precursor. It is widely used in the manufacture of formaldehyde, chloromethans, acetic acid, acetic anhydride, methyl formate and dimethyl ether. Methanol also finds increasing use as an octane booster for gasoline by direct blending or as a raw material for methyl tert-butyl ether(MTBE) and for fuel cell application. Furthermore, there are the exciting discoveries that methanol can be converted to high octane gasoline(Mobil methanol-to-gasoline process) and olefins(UOP/Hydro methanol-to-olefins process).
All the industrial methods for producing methanol are very similar to each other and are based on two fundamental stages namely a first stage in which the raw material is converted into synthesis gas (CO/H.sub.2 /CO.sub.2 mixture) and a second stage in which the CO/H.sub.2 /CO.sub.2 mixture is converted into methanol with heterogeneous gaseous phase catalysis. The industrial operating conditions for latest generation copper catalysts are a pressure of 5-10 MPa, a temperature of 230-270.degree. C. and an (H.sub.2 --CO.sub.2)/(CO+CO.sub.2) of 5/1-8/1(by volume).
The relatively low conversion per pass and the consequent need to maintain a low inert gas (N.sub.2, CH4 etc.) content in synthesis gas are the main limitation of current technology.
Catalyst systems operating under very mild temperature and pressure conditions (90-150.degree. C., 1-5 MPa, respectively) have recently been developed. With these it is possible to obtain a very high CO conversion of greater than 90% per pass, to thus well overcome the main limitation of current technology. Many of these catalytic systems use nickel as the catalyst metal catalyst. All these systems have however the drawback of forming nickel carbonyl, a very toxic substance, under reaction conditions. A promising alternate metal is copper which does not form highly toxic substances under the low-temperature synthesis conditions. Furthermore when using copper as the catalyst metal, more methyl formate can be obtained.
Recently, methyl formate has been considered as a building block in the synthesis of various chemicals from syngas. An integrated chemical industry complex involving methyl formate could come into existence in the future. The number of reactions that convert methyl formate to other chemicals is large. In particular, the synthesis of large volume chemicals such as acetic acid, ethylene glycol and methyl methacrylate(MMA) deserve serious consideration. Compared to the conventional synthesis from syngas, the methyl formate routes usually require much milder reaction conditions, for example, in the process for producing acetic acid by the isomerization of methyl formate.
For manufacturing the large tonnage of commodity chemicals, it is necessary that methyl formate should be produced at much lower cost than that of the methanol carbonylation process.
Evidence showing that the low-temperature synthesis reaction can occur at 200.degree. C., and 15-25 MPa with sodium carbonate or sodium formate in combination with a copper-chromium-calcium catalyst was provided.
Aker Engineering, in Petrol Engineering, reported a two-component liquid phase catalytic system to convent syngas to mixture of methanol and methyl formate in a single reactor. The process was reported to operate typically at 110.degree. C. and 0.5 MPa. The report disclosed the use of only alkali and/or alkaline earth alkoxides(alcoholates) as the carbonylation catalyst with copper chromite as the hydrogenolysis catalyst. The report emphasized the need to eliminate all CO.sub.2, H.sub.2 O and sulfur compounds from the inlet syngas.
Similarly, U.S. Pat. No. 4,731,386 discloses preparation of methanol from syngas in a liquid reaction mixture in the presence of a catalyst system consisting of an alkali alcoholate and a heterogeneous copper catalyst. It was found that the addition of a non-polar organic solvent having weak cation solvating properties in the liquid phase, otherwise, consisting of methanol and methyl formate, substantially increased the catalytic activity of catalyst systems consisting of an alkali metal alcoholate and a heterogeneous copper catalyst.
More recently, U.S. Pat. No. 5,384,335 discloses a low-temperature process for the synthesis of methanol from syngas under relatively mild conditions in a slurry phase reactor with a catalyst combination comprising reduced copper chromite and basic alkali salts or alkaline earth salts. The invention allows the synthesis of methanol to occur in the temperature range of approximately 100.degree.-160.degree. C., and the pressure range of 4.0-6.5 MPa. The process produces methanol with up to 90% syngas conversion per pass and up to 95% methanol selectivity. The only major by-product is a small amount of methyl formate. Although the catalytic system of the U.S. Patent shows interesting characteristics such as its capacity to produce methanol by using the basic alkali salts or alkaline earth salts as a catalytic component under relatively mild reaction conditions, it has the limitation of low productivity, this being a important limitation from the applied viewpoint. In particular, the low-temperature(.ltoreq.120.degree. C.)activity of the catalytic system of the U.S. Patent is very low. Therefore the selectivity to methyl formate is also low when using the catalyst combination.
The object of present invention is to overcome the disadvantages in prior art, namely the high temperatures needed to synthesize methanol via the conventional gas-phase process using the copper-zinc catalysts and the low activity of using commercially available copper chromite as the catalyst component and to provide a high-activity copper catalyst and a highly efficient catalyst combination for the low-temperature co-production of methanol and methyl formate which enable the synthesis of methanol and methyl formate under mild temperatures while can be achieved high syngas conversion per pass and high productivity of methanol and methyl formate.